sad, sadly

Sad or sad describes an unhappy or sorrowful person, thing, state or condition--
Do you feel sad?
This is a sad song.
Sad may also mean deplorably bad; pitiful--
This is a sad state of affairs.

Sadly or sadly describes an unhappy or sorrowful action--
"It's raining", she said sadly.
The mourners moved sadly and slowly, in single file.

Now that you know that, you can say--
"I didn't realize how sad this poem was, unitl I heard him recite it so sadly."